Small monitors mostly come from consistently hot places, relatively extreme environments. Unless you lived in the right climate, an outdoor enclosure would be tricky to get right. I certainly wouldn't be trying in Melbourne or Sydney - largely for the same reasons large monitors naturally live there but small monitors don't, small monitors aren't likely to do well. Even in the right climate, they do best with fairly closely controlled feeding, so indoor enclosures work best; you can closely monitor your monitors and keep everything right. If you really want a small to medium lizard to keep outdoors, there are many dragons and skinks well suited to it.